ABSTRACT The long term goal of this study is to understand the molecular basis of genomic instability and its role in cancer. The three aims in this proposal are motivated by our previous study [1] and from a recently published study in mammalian cancer cells. We have used the model organism Saccharomyces cerevislae (budding yeast) to develop a specific chromosome assay system (referred to as the ChrVII system in this proposal). We have previously used this system to identify a novel unstable chromosome site (a fragile site) that when unstable leads to the formation of unstable chromosome intermediates. We have identified roles of key genes in stability of the fragile site, and have developed a working model that explains its instability. In this proposal, we first will continue to identify genes and genetic pathways required to stabilize the specific unstable site, using both a candidate gene and a genome wide genetic screen. Second, we will identify additional genetically unstable sites in the yeast genome, which will lead to an understanding of the multiple chromosomal determinants that cause instability. Third, we will test one hypothesis for why unstable chromosome intermediates are unstable. This one hypothesis is motivated in part by recent observations in cancer cells that suggest to us that unstable chromosomes are common, for unknown but potentially important reasons, in yeast and in mammalian cells (and thus probably in all cell types). Each of these three aims is of interest in cancer research, and each is technically demanding enough that detailed study in all but the most sophisticated model organisms is cumbersome at best. Budding yeast has a sophisticated yet easily manipulated genome, and as such is useful to carry out the experiments proposed that are of general relevance in chromosome biology and cancer. This project will also provide an excellent educational and training vehicle for Native American students in cancer biology, and thus will contribute strongly to the goals of The Partnership for Native American Cancer Prevention to alleviate the unequal burden of cancer among Native Americans.